Replacement of natural hardwood forest with planted bamboo and cedar in a humid subtropical mountain affects soil microbial community

Reforestation with different types of plants can affect soil biochemical activity and microbial community structure. However, little is known about the mechanism by which reforestation affects soil microbial community. We investigated soil microbial communities and biochemical properties in moso bamboo and Japanese cedar plantations in central Taiwan, which were established through reforestation of a natural hardwood forest. Soil microbial biomass carbon (Cₘᵢc) and nitrogen (Nₘᵢc) were lower in two reforested plantations than in natural hardwood forest. The conversion of natural hardwood forest to cedar and bamboo plantations, however, increased soil Cₘᵢc/Cₒᵣg and Nₘᵢc/Nₜₒₜ ratios and decreased soil metabolic quotient (respiration per unit of microbial biomass). Most soil enzymatic activities decreased after the shift from natural hardwood forest to reforested plantations. The abundance of total phospholipid fatty acids (PLFA), bacteria and gram-positive bacteria was lower in bamboo and cedar plantations than in natural hardwood forest. However, ratios of soil gram-positive to gram-negative bacteria and cyclopropane fatty acids to monoenoic precursor fatty acids, which are considered indicators of physiological stress, were lower in the reforested plantations as compared to the natural hardwood forest. These results suggest that the microbial community is less physiologically stressed in the bamboo and cedar plantations soils, though it still had a smaller biomass. Principal component analysis of soil PLFA content showed that the natural forest separated from bamboo and cedar plantations. Similarly, denaturing gradient gel electrophoresis analysis revealed changes in bacterial and fungal community structures after the reforestation. Our results suggest that the reforestation with bamboo and cedar in the strongly acidic natural forest drove the microbial community structure to favor gram-negative bacteria, possibly due to an increase in nutrient availability after the forest conversion.